There are several types of prior art dispensing systems used for dispensing metered amounts of liquid or paste for a variety of applications. One such application is in the assembly of printed circuit boards and integrated circuit chips. In this application, dispensing systems are used in the process of encapsulating integrated circuits with an encapsulating material and in the process of underfilling flip integrated circuit chips with an encapsulent. Prior art dispensing systems are also used for dispensing dots or balls of liquid epoxy or solder paste onto circuit boards and integrated circuits. The liquid epoxy and solder are used to secure components or connect components, respectively, to a circuit board or to an integrated circuit. The dispensing systems described above include those manufactured and distributed by Speedline Technologies, Inc, the assignee of the present invention, under the name CAMALOT™.
The dispensing systems described above are typically used in an electronics manufacturing facility in an automated assembly line with other equipment used in a circuit board or integrated circuit manufacturing process. The other equipment in-line with the dispensing systems may include, for example, pick and place machines, which place dispensing systems may include, for example, pick and place machines, which place components on circuit boards, or reflow ovens that are used to heat materials, such as solder paste, dispensed onto the circuit boards or integrated circuits.
In a typical dispensing system, a pump and dispenser assembly is mounted to a moving assembly for moving the pump and dispenser assembly along three mutually orthogonal axes (x, y, z) using servomotors controlled by a computer system or controller. To dispense a dot of liquid on a circuit board at a desired location, the pump and dispenser assembly is moved along the horizontal x and y axes until it is located over the desired location. The pump and dispenser assembly is then lowered along the vertical z axis until the nozzle of the pump and dispenser assembly is at an appropriate dispensing height over the board. The pump and dispenser assembly dispenses a dot of liquid, is then raised along the z axis, moved along the x and y axes to a new location, and is lowered along the z axis to dispense the next liquid dot.
During the manufacture of circuit boards, it is sometimes necessary, or desirable, to dispense two different liquids or pastes onto a circuit board or to dispense different volumes of the same material. Dispensing systems have been designed that can dispense one of a number of dispensing materials from one dispensing head. One example of such a dispensing system is described in U.S. Pat. No. 5,795,390 issued Aug. 18, 1998 to Cavallaro, which is incorporated herein by reference. These dispensing systems typically are only able to dispense one material at a time, and the throughput of product in these systems may be less than desired because of the time required to dispense multiple materials serially using one dispensing head.
To overcome the throughput problem, two dispensing systems may be placed adjacent to each other with the first dispensing system dispensing one material and the second dispensing system dispensing a second material. This solution is expensive since two complete machines are used, and since additional manufacturing space is required. In typical operations, manufacturing floor space is limited, and it is desirable to limit the “footprint” of each manufacturing system on the manufacturing floor.
Printed circuit boards continue to become more densely populated, and circuit components continue to become smaller. As a result, a need has developed for dispensing systems that are able to accurately dispense dots of material, such as solder paste or adhesive, having diameters less than 0.010 inches (10 mils). To dispense small dots of material, it is known to use an auger style pump, such as that disclosed in U.S. Pat. No. 5,819,983 issued Oct. 13, 1998 to White et al., which is incorporated herein by reference. Auger style pumps such as these, typically feed the auger with material using pressurized air, and dispensing from the auger occurs by rotation of the auger by a DC motor. A needle is attached to the output of the auger and the material is dispensed out of the tip of the needle. In typical prior art auger style pumps, auger acceleration is a function of characteristics of the clutch used, motor torque, packing washers, and friction, and auger deceleration is a function of the packing washers and friction. Therefore, the acceleration and deceleration of these pumps is not well controlled, making it difficult to accurately control dispensing. Therefore, these pumps are typically not used to dispense small dots having diameters less than 0.010 inches.
As discussed in a technical paper titled The Dawn of Micro-Dispensing, by Russ Peek, SMT Magazine, July 2001, a known rule of thumb in the electronics industry is that the minimum dot diameter a needle can deliver is equal to the inside diameter of the needle multiplied by 1.5. Based on this rule, the inner diameter of a needle would have to be 3.3 mils to dispense a dot having a diameter of 5 mils. However, for dispensing of solder pastes, silver epoxies and other adhesives in the electronics industry, when a needle with an inner diameter of less than approximately 5 mils is used, clogging of the needle often occurs due to the size of particles contained in these materials. The clogging of the needle causes unrepeatable dispensing results.